It can fluctuate from tank to tank. I get awful mileage if I drive the way I want to. I have to make sure I shift before boost engages, and even then I get around 19-20. In the winter I regularly get 17. 16.5 is not far off of that. The MPG is based off of the trip so it shouldn't matter if you reset it at a stop or rolling, honestly. It's based off of the amount of throttle you're giving the car compared to the speed, calculating load. Anyway, once the car has completed a full learning cycle (typically up to a hundred miles) it should adjust to your driving habits and elevation. With it being that new I wouldn't be surprised if it still had some learning to do. I heard that new ECUs have a preprogrammed distance before they will fully learn. I only heard that though, and never read it anywhere official.